MA + ), formamidinium (FA + ) and X = I − , Br − , Cl − . Furthermore, they are classified by their perovskite crystal structure (Figure 1a). Their lower toxicity compared to their more established APbX 3 counterparts and relatively narrower yet tunable bandgap make them attractive candidates for a range of optoelectronic applications. [5] However, the facile oxidation of the Sn(II) species to Sn(IV) compounds severely limits their lifetime, especially under ambient conditions. [6] The development of Sn-based semiconductors necessitates a reliable determination of the chemical state of the constituent elements. X-ray photoelectron spectroscopy (XPS) is an important tool in this regard to conduct research on novel energy materials and devices. [7,8] Typically, these studies are based on the observation of the core level photoelectron emission features and their position on the binding energy (E b ) scale. To counteract charging effects, referencing based on adventitious carbon is often performed. However, this results in a typical inaccuracy of ±0.2 eV, whereas even errors of >1 eV have been observed. [9] For many Sn compounds with different Sn oxidation states and compositions, the main Sn 3d core level emission exhibits only minimal E b shifts, which lie below this error range. Consequently, probing degradation as well as assigning the effects of compositional changes and additives to the surface chemistry of Sn-based perovskites with conventional XPS can be challenging.In addition, shifts on the binding energy scale not originating from chemical bonds are especially pronounced for semiconductors (Figure 1b). Similar to effects observed in UPS, [10] excitation during measurement with an X-Ray source may induce photovoltages, [11] resulting in surface band bending. Surface charges, e.g., induced by ionic additives, may lead to similar effects that originate from the resulting space charge and not the bonding to the additive itself. [12] Consequently, a probing depth-dependent shift is induced, which may further inhibit reliable charge referencing. Similarly, changes in the materials' work function (e.g., due to differences in doping) would also result in changes of the absolute binding energies for an otherwise identical material.For chemical state analysis of Sn, XPS analysis based on the modified Auger parameter (AP) concept offers an effective alternative to distinguish different chemical states. [13] This approach relies on the observation of both the photoelectrons (i.e., Sn 3d) and Auger electrons (i.e., Sn MNN).
Reliable chemical state analysis ofSn semiconductors by XPS is hindered by the marginal observed binding energy shift in the Sn 3d region. For hybrid Sn-based perovskites especially, errors associated with charge referencing can easily exceed chemistry-related shifts. Studies based on the modified Auger parameter α ′ provide a suitable alternative and have been used previously to resolve different chemical states in Sn alloys and oxides. However, the meaningful interpretation of Auger parameter var...
